Chemistry Chapter 7

• An exothermic reaction is a reaction that transfers energy to the surroundings.
• Exothermic Reactions feel hot.
• Disposable hand warmers are a good example of endothermic reactions, they transfer energy to the surroundings and they are warm so therefore they are an exothermic reaction.
• More examples of exothermic reactions are; Fire; Compost heap; Cement and Barbed wire.
• This is an exothermic reaction profile it shows, as the reactnats are made together, it transfers energy to make the products.

• An endothermic reaction is a reaction that takes in energy from the surroundings.
• Endothermic reactions feel cold.
• A good example of an endothermic reaction is an ice pack because in an ice pack, it contains ammonium nitrate and water. As the ammonium nitrate dissolves, it takes in energy from the surroundings therefore making it colder.
• This is an endothermic reaction profile, this is because, as the products are made together, energy is absorbed to make the products.

• In an exothermic reaction, more energy is released by the product than the amount of energy that is used to break the bonds.
• An an endothermic reaction, less energy is taken in by the product than the amount of energy used to create the bonds.

2H2 + O2 = 2H2O

H - H = 436

O - O = 498

(436 x 2) + (1 x 498) = 1370 kj/mol (energy required to break the bonds

4 x 464 = 1856 kj/mol

1370 - 1856 = -486kj/mol (energy to make bonds)

Advantages of hydrogen fuel cells:

• Do not need to be electrically recharged
• No pollutants are produced
• Can be a range of sizes for differet uses

Disadvantages for hydrogen fue cells;

• Hydrogen is highly flammable
• Hydrogen is sometimes produced for the cell by non - renewable resources
• Hydrogen is difficult to store

A more efficient use of the energy from oxidising hydrogen is in a fuel cell. hese cells are fed with hydrogen and oxygen and produce water. Most of the energy released in the reaction is transferred to electrical energy. This can be used to run a vehicle, however, a constant supply of hydrogen is still needed to run the fuel cell.

• Metals tend to lose electrons and form positive ions.
• When two metals are dipped in a salt solution and joined by a wire, the more reactive metal will donate electrons to the less reactive metal. This forms a simple electrical cell.
• The greater the difference in reactivity between the two metals, the higher the voltage produced by the cell.